The present invention relates to corrosion control for recirculating hot water systems, and pertains particularly to a system and method for removal of corrosive particles and gases from recirculating hot water systems.
Corrosion has been a major problem for recirculating hot water systems for some time. Failure due to corrosion occurs in many forms and under many different conditions. For this reason, there has been continuous debate as to the causes of the corrosion problems.
One major form of corrosion that leads to failure of pipe systems is that of impingement corrosion. This type of corrosion occurs where gases and solids in liquids impinge on the metal surfaces of the plumbing at high velocities. Rapidly moving bubbles (air or gas) and particles (grit and scale) can strip away the naturally occurring protective films that occur and protect copper piping. When this occurs, the metal corrodes at a more rapid rate in an effort to reestablish the protective film. However, because the film is being swept away as fast as it is being formed, the corrosion proceeds at a high rate, leading to early failure.
There are basically two forms of impingement corrosion. A first form is erosion-corrosion, which is caused by solids (grit and scale) in liquids mechanically abrading the surface of the metal. A second form is cavitation caused by gas bubbles, which forms and collapses in the liquid in response to pressure and directional changes in the liquid. The collapse of the bubbles exerts forceful hammer-like blows on the metal surface.
The heating of water drives out of solution most of the air and gas therein, which has a high percent of carbon dioxide (CO.sub.2), which is itself destructive to copper. Dissolved gases are a major factor contributing to the corrosion of metal.
Corrosion is an electro chemical phenomenon. It increases with increase in temperature. Increasing temperature reduces the solubility of most gases, including oxygen and carbon dioxide (CO.sub.2), such that they are released as temperature increases. For a closed system in which oxygen and CO.sub.2 cannot escape, corrosion continues to increase at least linearly with increase in temperature. Other gases may also be present in water which are also undesirable. These, for example, may be volatile organic compounds which are desirable to remove from water. Carbon dioxide is a gas that is more soluble than oxygen and will convert to carbonic acid, producing a solution where acid attack, and the subsequent lowering of Ph, can dominate, creating a more corrosive condition to metals.
Gases trapped in water also, as previously explained, cause cavitation. This occurs in moving water where the flow is disturbed so as to create local pressure drops. Under these conditions, a vapor bubble will form, then collapse, applying a momentary stress of up to 1379 MPa (200 ksi) to the surface. Current theories is that this repeated mechanical working of the surface creates a local fatigue situation that aids the removal of metal. Impingement attack can be reduced and the life of the unit extended by decreasing fluid velocity, streamlining the flow, and removing entrained air. In the past, this is usually accomplished by designing water boxes, injector nozzles and piping to reduce or eliminate low pressure pockets, obstructions to smooth flow, and abrupt changes in flow direction, and other features that cause local regions of high velocity or turbulent flow.
Impingement corrosion can also result from the presence of hard water which predominates throughout the United States. Hard water is a widespread problem in the United States as ground water percolates through limestone rock and dissolves calcium and magnesium minerals, which remain in solution, creating hardness measures, in grains per gallon. According to the Water Quality Association, eighty-five percent of the potable water supplied in the United States is to some degree hard, based on the contents of dissolved minerals in the water. When hard water is heated, the minerals precipitate out of solution and quickly solidify into hard scale or turning back into limestone. Scale layers can build up, choking hot water pipes, fouling thermostats and encrusting hot water heater elements, etc. These scales also form particles that are suspended in the water and carried along with it, resulting in impingement of the walls of the water system, and ultimate erosion thereof.
Many attempts and proposals have been made in the past to remove these solids and gases. However, these attempts have not been satisfactory.
It is, therefore, desirable that an improved system and method be available for the elimination of these corrosive agents from hot water systems.